Typically, electrical connections (potheads) to electrical submersible pumps (ESPs) used in oil wells are made by installing a cured rubber grommet with a jamb nut threaded into the face of the pothead. This causes the rubber seal member to tighten against the insulated conductor and effect a seal. When the pothead is placed in service, the temperature increases due to the elevated temperature of the oil well and the added temperature rise of the motor. Because the coefficient of thermal expansion of rubber is approximately 10 times more than that of steel, the pressure in the seal area increases dramatically with the increasing temperature. This elevated pressure causes the cable insulation to reduce in diameter and gradually extrude out of the seal area. If the pump is operated over an extended period of time under these conditions, as is usually the case, the deformed insulation will take a permanent set, resulting in a necked-down conductor insulation in the seal area. When the temperature is reduced, as when the pump is turned off for a short while, the seal member contracts and no longer fits tightly against the insulation of the conductor. As a result, the pothead fails to prevent the ingress of contaminants into the motor, and can ultimately lead to a motor or pothead electrical failure.
Many people have recognized this problem, and a variety of solutions have been proposed, such as urging a movable wall of the seal cavity with springs so that as the seal expands and contracts, the amount of pressure rise will be restricted and, at least in theory, the seal will be maintained against the insulation even after this insulation has deformed and taken a permanent set in its reduced diameter configuration in the seal region.
This common approach to maintaining an effective seal fails to account for a very fundamental mechanism that takes place when the insulated conductors becomed necked-down due to their having taken a permanent set. It has been observed that the reduction of insulation diameter in the seal region is uniformly distributed about each individual conductor. Hence, when the temperature of the device is reduced, the rubber seal must be shortened in length to reduce the diameter of the holes in the seal plug to maintain a seal. Most spring-urged concepts employ a movable disk on one side of the seal plug, which imposes a uniform movement of one surface of the rubber plug. The cross-sectional area of the rubber seal in contact with the outer portions of the insulated conductors is far greater than the cross-sectional area of the rubber seal in contact with the center portions of the insulated conductors. Calculations indicate that to maintain contact of the rubber seal with the reduced diameter insulation in the seal region, five times more reduction in length of the center plug region is required than the outer portions of the plug. A spring-urged movable disk cannot cause such a movement of the center portion of the seal plug with respect to the outside portions. Hence, leaks consistently occur in the center region after temperature cycling, even when sliding disks and springs are used.
This invention addresses this problem in the art, along with other needs which will become apparent to those skilled in the art once given this disclosure.